I used to use pressure barrels, and found they were excellent for low carbonated beers like Bitters. The CO2 cartridges kept the carbonation consistent. If you don't use the CO2, as David said, the beer will get flatter, and as you can probably drink faster than the CO2 comes out of solution, you will end up with a partial vacuum in the barrel which will prevent you from drawing off more beer unless you release the vacuum by letting air in. That is like pouring a glass of beer, and then leaving it for several days before drinking it.
As for keeping beer "fizzy", I never tried it, so I don't know how well it would work.